Heat-softenable toners are widely used in imaging methods such as electrostatography, wherein electrically charged toner particles are deposited imagewise on a dielectric or photoconductive element bearing an electrostatic latent image. Most often in such methods, the toner is then transferred to a surface of another substrate, such as, e.g., a receiver sheet comprising paper or a transparent film, where it is then fixed in place to yield a final desired toner image.
When heat-softenable toners, comprising for example thermoplastic polymeric binders, are employed, the usual method of fixing the toner in place involves applying heat to the toner once it is on the receiver sheet surface to soften it, and then allowing or causing the toner to cool.
One such fusing method comprises passing the toner-bearing receiver sheet through a nip formed by a pair of opposing members, typically in the form of cylindrical rollers, wherein at least one of the members (usually referred to as a fuser member) is heated and contacts the toner-bearing surface of the receiver sheet in order to heat and soften the toner. The other member (usually referred to as a pressure member) serves to press the receiver sheet into contact with the fuser member. In some other fusing methods, the configuration is varied and the “fuser member” or “pressure member” can take the form of a flat plate or belt.
The desired gloss of the fused electrostatographic images can vary depending on the thermoplastic binder used for the toner, the materials used for the surfaces of the fuser and/or pressure members, and conditions employed during the fusing step as mentioned briefly hereinafter. Typically, it is preferred that multicolor pictorial images have a glossy finish and monochromatic text and graphics have a matte finish.
Several methods for imparting glossy or matte finishes to an image have been disclosed. One method is to cover a multicolor toner image with clear, glossy toner. The clear toner can be laid down in an image configuration or it can be laid down uniformly over the whole image. See, for example, Crandall, U.S. Pat. No. 4,828,950 and Ng, U.S. Pat. No. 5,234,783.
Another method to provide glossy pictorial toner images, produced in an undercolor removal apparatus, is to lay a black matte toner down first and completely cover it by a color (cyan, magenta, yellow) toner having a more glossy finish after fusing. Examples of such methods are described in Japanese Patent Application No. 133422/87, Laid Open No. 300254/88, Dec. 7, 1988. Additional references which disclose the use of glossy and matte toner combinations include Japanese Patent Application No. 90JP-333829, Laid Open No. C92-132261, and U.S. Pat. Nos. 5,162,860 and 5,256,507.
The use of different fuser rollers or finishing apparatus to effect the gloss of a fused toner image has been considered. It has been disclosed that hard metallic rollers covered with a fluorocarbon resin can be used to produce fused toner images having high gloss. On the other hand, most soft rubber coated rollers impart a matte finish to fused images.
U.S. Pat. No. 5,118,589 discloses the use of pressure members with a predefined surface finish to impart either gloss or texture to a heat softenable layer of a receiver onto which color toner particles have been thermally transferred. The use of textured pressure members to impart texture to fixed toner images has also been disclosed in U.S. Pat. Nos. 4,258,095 and 5,085,962. U.S. Pat. No. 5,019,869 discloses an electrophotographic device in which a finish is applied to a toner image by selecting one of a plurality of finishing rollers, each roller having a different and distinct surface texture. Further, U.S. Pat. No. 5,319,429 illustrates the use of a fusing apparatus comprising two endless belts each having a glossy surface to provide glossy images.
U.S. Pat. No. 4,639,405 discloses an apparatus for providing glossy fused toner images which passes toner-bearing receivers sequentially through a first and second pair of rollers, the first pair of rollers fuses the toner, and the second pair of rollers provides gloss to the toner image.
Another method for affecting the gloss of an electrophotographic image is to change the toner binder resin rheology, and therefore, the melt flow characteristics of the toner composition. A toner which has higher melt flow properties at a given temperature, provides higher image gloss as compared to a toner formulation which has lower melt flow properties. Because the melt viscosity of a polymer changes as a function of the weight average molecular weight, substantial changes in the melt viscosity of a toner can be achieved by controlling the molecular weight of the toner binder. References which disclose that changing the molecular weight can affect the gloss include U.S. Pat. Nos. 4,913,991 and 5,258,256.
The amount of crosslinking in the toner binder polymer also can affect gloss. Typically, toners having high crosslinked polymer binders provide matte images. An example of such toner for the purpose of providing a low gloss image is detailed in U.S. Pat. No. 5,395,723.
U.S. Pat. No. 5,334,471 teaches a method of controlling gloss in an electrophotographic toner image by utilizing light-scattering particles of a specific size range. The light-scattering particles are large enough to provide a bumpy image surface which is said to impart low gloss.
As described above, in electrostatographic processes using toners, matte or glossy finishes of the fused toner image can be provided either by controlling the rheological behavior of the toner binder polymer or by controlling the surface texture of the fusing members. However, even with these materials, it has generally not been possible to easily adjust the gloss capabilities of the fuser member by a simple adjustment of the materials used to make the fuser member. It would be desirable to have an ability to make a fuser member having a pre-determined gloss for the fusing surface, as this would enable such fuser member to fuse toner images to a desired gloss specification.
Therefore, as can be seen, a need exists for improved compositions, methods and apparatus to produce fused toner images which meet a pre-selected gloss level.